CONFIRMATION VIA THE CONTINUUM-FITTING METHOD THAT THE SPIN OF THE BLACK HOLE IN CYGNUS X-1 IS EXTREME

• Main Authors: Gou, Lijun (Author), McClintock, Jeffrey E. (Author), Steiner, James F. (Author), Reid, Mark J. (Author), Orosz, Jerome A. (Author), Narayan, Ramesh (Author), Hanke, Manfred (Author), Garcia, Javier A. (Author), Remillard, Ronald A (Author)
• Other Authors: MIT Kavli Institute for Astrophysics and Space Research (Contributor), Remillard, Ronald Alan (Contributor)
• Format: Article
• Language: English
• Published: Institute of Physics/American Astronomical Society, 2015-02-18T21:57:19Z.
• Subjects: Article
• Online Access: Get fulltext

In Gou et al., we reported that the black hole primary in the X-ray binary Cygnus X-1 is a near-extreme Kerr black hole with a spin parameter a * > 0.95 (3σ). We confirm this result while setting a new and more stringent limit: a * > 0.983 at the 3σ (99.7%) confidence level. The earlier work, which was based on an analysis of all three useful spectra that were then available, was possibly biased by the presence in these spectra of a relatively strong Compton power-law component: the fraction of the thermal seed photons scattered into the power law was f s = 23%-31%, while the upper limit for reliable application of the continuum-fitting method is f s lsim 25%. We have subsequently obtained six additional spectra of Cygnus X-1 suitable for the measurement of spin. Five of these spectra are of high quality with f s in the range 10%-19%, a regime where the continuum-fitting method has been shown to deliver reliable results. Individually, the six spectra give lower limits on the spin parameter that range from a * > 0.95 to a * > 0.98, allowing us to conservatively conclude that the spin of the black hole is a * > 0.983 (3σ).
Germany. Bundesministerium für Bildung und Forschung (grant No. DLR 50 OR 0701)
National Natural Science Foundation (China) (Y211541001)
National Natural Science Foundation (China) (NSFC grant 11333005)
National Astronomy Observatory of China (NAOC grant (Y234031001)
Chinese Academy of Sciences (Strategic Priority Research Program "The Emergence of Cosmological Structures", grant No. XDB09000000)
United States. National Aeronautics and Space Administration (NASA grant NNX11AD08G)
United States. National Aeronautics and Space Administration (NASA Hubble Fellowship grant HST-HF-51315.01)